1. Field of the Invention
This invention relates to a method of fabrication of lead frames for use in the fabrication of semiconductor devices.
2. Brief Description of the Prior Art
Lead frames are a standard component utilized in the fabrication of semiconductor devices. In general, during the fabrication of semiconductor devices, semiconductor chips are secured to a central portion of the lead frames with fine gold wire leads bonded to and extending from bond pads on the chip to lead fingers on the lead frame. The lead frame and chip are subsequently encapsulated with a portion of the lead fingers of the lead frame extending externally of the encapsulation for connection to the external environment, such as, for example, a printed wiring board (PWB). Lead frames having relatively large dimensions and/or a relatively low pin count have been produced from metal sheets by die punching. However, with the continuing decrease in the pitch available between leads, lead frames have been fabricated by use of a masking and etching of an electrically conductive thin sheet or foil as discussed in U.S. Pat. No. 5,221,428. Although current metal punching technology has achieved capability nearly equal to that of etching, the latter is still favored for new designs and low volume production due to the high cost of such metal punching tools, especially for very high pin count lead frames.
To fabricate a lead frames by etching, a photo resist pattern in the shape of the lead frame to be fabricated is applied to both sides of an electrically conductive metal, such as, for example, a foil, generally copper or a copper-based material or low expansion iron-nickel alloy, such as ALLOY 42. This foil with patterned photo resist thereon is then passed through an etcher wherein the metal which is not masked by the photo resist is removed by etching, such as, for example, with ferric or cupric chloride. After etching, the photo resist is stripped away and the lead frame is optionally ready for partial or complete plating to improve bonding and soldering properties of the metal, such plating, if utilized, generally being palladium or selective silver or gold. During this etching process, the foil is also subjected to underetching or undercutting wherein, as the metal foil is etched through, the pattern that is etched away is somewhat larger than the open areas in the original resist pattern due to sideways etching under the resist. The amount of undercutting is related to the thickness of the foil and increases with increase in foil thickness. As noted above, this effect is caused by the fact that the etchant is not only etching downward or normal to the plane of the foil surface, but also, at a somewhat slower etch rate, is also etching sideways or into the plane of the foil. In standard fabrication techniques, this effect is compensated for by biasing the photo resist pattern. However, if very fine slots are being fabricated in a relatively thick foil, as is now required for lead frames used in conjunction with the small geometries now required in semiconductor fabrication, it is necessary to reduce the width of the slots in the resist to the extent that they theoretically disappear. Since this is not a possible scenario, there exists a limit to the minimum slot width that can be etched for a given foil thickness. It is therefore apparent that new techniques are required whereby narrower slots can be etched than are available with prior art techniques for a given foil thickness. Such a new technique is et forth in U.S. Pat. No. 5,221,428 wherein, the undercutting of the prior art is reduced by reducing the thickness of selected portions of the foil and etching from both major surfaces. While undercutting is reduced, sideways etching still takes place at about half the rate as does etching normal to the plane of the foil.
In accordance with the present, the above problem of undercutting is minimized.
Briefly, in the external lead finger region of the lead frame, the assembly of the integrated circuit as well as the later assembly of the integrated circuit onto a printed circuit board requires a certain mechanical strength that can only be obtained by having a known minimum material thickness. Typically, only the inner leads that are within the plastic encapsulation have a very fine pattern with very narrow slots. In the outer area, the gaps are much wider. The prior art solution was to reduce the material thickness in the internal lead area by etching down in a separate pass through the etching process. The etching was accomplished from one or both sides of the foil, preferably from both sides of the foil. After the initial etching step was completed, the standard etching process for etching lead frames was then incorporated with a photo resist being used to reliably cover the vertical sides of the etch cavity. This prior art solution is altered in accordance with the present invention by stopping the etching of the foil after deposition of the first photo resist as soon as the material thickness that is left is sufficiently thin to enable the production of the fine features sought. The first photo resist is then stripped, the partially etched foil is cleaned in standard manner and a second, preferably liquid, photo resist is now applied which covers the foil and the cavities that were etched in the foil during the prior etching process. The second liquid photo resist can be applied from one or both sides of the foil. The second photo resist is then patterned and exposed, leaving the side walls along the cavities formed masked with the second exposed photo resist. After developing of the second photo resist, the substrate is again etched with the undercutting now minimized due to the presence of the second photo resist on the side walls. Accordingly, the undercutting is minimized. The second photo resist is then removed and processing continues in standard manner.